For example, in the hydraulic apparatus of the vehicular brake system, for the purpose of pressure accumulation or pulsation damping, there is employed an accumulator in which a pressure sealed chamber and a pressure flow-in chamber are formed by an operation member including a flexible metallic bellows. The accumulator is structured such as to damp and absorb a vibration by balancing a pressure in the pressure sealed chamber determined by the expansion and contraction of the metallic bellows and a pressure in the pressure flow-in chamber determined by the flow-in of the pressure fluid from the system side, and holds a high-pressure gas sealed within the pressure sealed chamber and the fluid sealed within the pressure flow-in chamber, and a gas end cover is installed in such a manner as to close an open end portion of a bottomed shell for fixing the metallic bellows.
In recent years, a weight saving of the vehicular parts is required for the purse of improving a specific fuel consumption or the like, and a weight saving of an accumulator 101 corresponding to the vehicular part is tried by reducing a thickness of a gas end cover 104 corresponding to a part of a constituting part of a housing 102. However, as shown in FIG. 3, since a thickness of a peripheral edge portion 105 of the gas end cover 104 is thin, there occurs necessity of largely curving an end portion of the peripheral edge portion 105 to an inner diameter side from a step concave portion 111 coupled to the shell 103 so as to secure a fixing portion 108 for fixing the metallic bellows 107. Accordingly, a dead space 109 is formed in the inner diameter side of the curved portion and a volume adjusting space 110 becomes large, so that there is a problem that a weight becomes heavy as well as an entire of the accumulator 101 becomes large in an axial direction.
Further, since the shape in which the end portion of the peripheral edge portion 105 is largely curved to the inner diameter side is employed for securing the fixing portion 108 as mentioned above, there is a problem that working steps for making the gas end cover 104 in the curved shape is increased.
Further, since the step concave portion 111 is formed in the curved portion as shown in FIG. 4, and the gas end cover 104 and the shell 103 are coupled at a weld portion 106 by aligning the step concave portion 111 and a terminal of the shell 103 and applying a carbon dioxide gas laser welding, a backing metal structure in which a spatter or the like is not scattered to an internal portion is employed. However, if the peripheral edge portion is formed in the curved shape, there occurs a problem that a thickness W1 of the backing metal portion becomes thin and there is a limit for securing a depth of weld penetration.
In this case, Japanese Unexamined Patent Publication No. 2002-122101 (Patent Document 1) proposes a structure in which a shape of a cross sectional center portion is formed in a convex shape to the pressure sealed chamber side and in a concave shape in the peripheral edge portion in order to secure strength in a state in which the thickness of the gas end cover is thin, however, since the peripheral edge portion is formed in the concave shape to the pressure sealed chamber side, the dead space exists, so that the structure cannot be effective for making the volume adjusting spacer small and making the working steps of the gas end cover easy.
Patent Document 1:
Japanese Unexamined Patent Publication No. 2002-122101